1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively performing a charging process with a charging power of a direct current voltage superimposed by an alternating current voltage.
2. Discussion of the Background
A background image forming apparatus employing an electrophotographic process includes a charging device constituting a charging mechanism configured to charge a photoconductor surface serving as an image carrier. One example charging device uses a proximity discharge method. In this charging device, a charging roller constituting a charging member is arranged to have a surface facing in close proximity to the photoconductor surface so as to form a discharge area with a minimum gap therebetween. The charging roller may be arranged in contact with the photoconductor surface. In this case, a discharge area is formed around a minute gap in vicinity to a contact part between the charging roller and the photoconductor surface.
In the charging device, obtaining a discharge amount capable of uniformly charging the photoconductor is known on an experimental basis and includes applying to the charging roller a direct current voltage and an additional alternating current voltage (i.e., a peak-to-peak voltage). The additional alternating current voltage is double a voltage Vth of the direct current voltage with which the charging roller starts a discharge to the photoconductor.
However, this known technique is suboptimal because the direct current voltage Vth may fluctuate due to variations in a resistance of the charging roller or a size of the gap caused by swelling of the charging roller due to an environment change. For example, Table 1 indicates a relationship between the direct current voltage Vth and an absolute humidity (AH) in units of g/cm3.
As a result of these fluctuations, the alternating current voltage (i.e., the peak-to-peak voltage) applied to the charging roller may drop below the value double the voltage Vth, and thereby the discharge to the photoconductor may not be performed, or the photoconductor may not be uniformly charged.
On the other hand, in a case where the alternating current voltage with more than a necessary amount is applied to the charging roller, an amount of the discharge to the photoconductor becomes excessive. This excessive voltage may deteriorate and whittle the photoconductor surface. As a result, a satisfactory image may not be maintained.
TABLE 1AH0 ≦5 ≦8 ≦18 ≦26 ≦AH < 5AH < 8AH < 18AH < 26AHVth20501840170016701640(volts)
Another example charging device includes an environment detection mechanism configured to detect temperature and/or humidity and attempts to adjust the alternating current voltage applying to the charging roller based on a result detected by the environment detection mechanism. However, this charging device requires an additional memory mechanism to be able to store the alternating current voltage corresponded to each environment such as temperature and humidity.
On the other hand, it has been known that the photoconductor surface is uniformly charged to a predetermined potential without being affected by the gap change when the alternating current value flowing to the photoconductor is equal to or above a predetermined value Ivth at a start of the discharge to the photoconductor. This predetermined value Ivth may be sought on an experimental basis and can be provided to the above-described example charging device as a reference. Thereby, the charging device can control the alternating current voltage value applying to the charging roller to equalize it with the predetermined value Ivth.
In particular, a predetermined alternating current voltage is applied to the charging roller during a warm-up time before an image forming operation is started, and a value of the alternating current is then detected. The alternating current value is determined whether or not equal to or above the predetermined value Ivth. In a case where the alternating current value is below the predetermined value Ivth, the alternating current voltage value is increased. After that, the alternating current value is again detected. Such operation is repetitively performed so that the alternating current voltage value capable of obtaining the reference alternating current value is set.